The attached-bit avoid-learning command disables a device from advertising default routes to a routing table when the ATT bit is set to 1.
The undo attached-bit avoid-learning command restores the device to generates default routes when the ATT bit is set to 1.
By default, a device advertises default routes when the ATT bit is set to 1.
An ATT bit is a field in link state packets. An ATT bit identifies whether Level-1 area is attached to other areas. A Level-1-2 device sets the ATT bit to 1 to notify the devices in Level-1 area of its attachment to a Level-2 backbone area. After a Level-1 device receives the LSPs carrying the ATT bit 1 from the Level-1-2 device, the Level-1 device generates a route with the same destination address as the default route of the Level-1-2 device. Traffic can be forwarded along this route.
To prevent the Level-1 device from advertising the default route when the ATT bit is set to 1, run the attached-bit avoid-learning command.
Run the attached-bit advertise never command on the Level-1-2 device to disable the device from sending LSPs with the ATT bit 1.
Run the attached-bit avoid-learning command on the Level-1 device that is connected to the Level-1-2 device.
The difference between the preceding commands lies in that the attached-bit avoid-learning command applies to a specified device.